Method and apparatus for predicting spot beam and satellite handover in a mobile satellite communication network

1. A satellite communication system adapted to transfer a call from a first spot beam to a second spot beam wherein the call is being transmitted between a satellite and a subscriber unit located in the first spot beam, the system comprising: a first processor, the first processor being adapted to: track a movement of the subscriber unit relative to a satellite based coordinate system; identify the second spot beam from among a plurality of spot beams; estimate a time at which to transfer the call that is dependent on the position of the subscriber unit relative to the satellite based coordinate system; and generate a signal to cause the satellite to transfer the call at the time; a second processor being adapted to respond to the signal generated by the first processor; a first antenna being associated with the satellite fore generating the first spot beam; and a second antenna being associated with the satellite for generating the second spot beam; wherein the first antenna and the second antenna are controlled by the second processor; and wherein the system is further adapted to transfer the call from a first satellite to a second satellite wherein the first processor is further adapted to (a) determine the position of the subscriber unit at the estimated time; (b) determine the position of the satellite at the estimated time; (c) calculate an angle of elevation using the position of the subscriber unit and the position of the satellite; (d) compare the angle of elevation to a threshold angle to determine whether the subscriber unit is moving out of the first footprint into a second footprint that is generated by the second satellite; (e) substitute a new time for the estimated time, if the angle of elevation is not less than the threshold angle and thereafter repeat steps (a)-(d); and (f) cause the first satellite to transfer the call to the second satellite when the angle of elevation is less than the threshold angle.

2. A method for determining when to transfer a call associated with a first spot beam to a second spot beam in a satellite communication system, wherein the call is being transmitted between, inter alia, a non-geostationary satellite and a subscriber unit located in the first spot beam, the steps comprising: tracking a movement of the subscriber unit relative to a satellite based coordinate system, estimating a transfer time at which to transfer the call based on the movement of the subscriber unit relative to the satellite based coordinate system, causing the satellite to transfer the call at the estimated transfer time, and determining when to transfer the call from a first satellite to a second satellite, wherein the call is being transmitted from the first satellite to a subscriber unit located in a first footprint, the first footprint being generated by the first satellite, and wherein the steps for determining when to transfer the call from the first satellite to the second satellite comprise: (a) determining the position of the subscriber unit relative to the satellite based coordinate system at the estimated time; (b) calculating an angle of elevation using the position of the subscriber unit and a position of the satellite; (c) comparing the angle of elevation to t threshold angle to determine whether the subscriber unit is moving out of the first footprint into a second footprint, the second foot print being generated by the second satellite; (d) substituting a new time for the estimated time, if the angle of elevation is not less than the threshold angle and thereafter repeating steps (a)-(c); and (e) causing the first satellite to transfer the call to the second satellite when the angle of elevation is less than the threshold angle.

3. A satellite communication system adapted to transfer a call from a first spot beam to a second spot beam wherein the call is being transmitted between a satellite and a subscriber unit located in the first spot beam, the system comprising: a first processor, the first processor being adapted to: track a movement of the subscriber unit relative to a satellite based coordinate system; identify the second spot beam from among a plurality of spot beams; estimate a time at which to transfer the call that is dependent on the position of the subscriber unit relative to the satellite based coordinate system; and generate a signal to cause the satellite to transfer the call at the time; a second processor being adapted to respond to the signal generated by the first processor; a first antenna being associated with the satellite fore generating the first spot beam; a second antenna being associated with the satellite for generating the second spot beam; a transceiver for receiving signals transmitted by the first processor and for routing the signals to the second processor; and a frequency translator for converting the signals received by the transceiver to a format suitable for controlling the first antenna and the second antenna; wherein the first antenna and the second antenna are controlled by the second processor.

4. A method for determining when to transfer a call associated with a first spot beam to a second spot beam in a satellite communication system, wherein the call is being transmitted between, inter alia, a non-geostationary satellite and a subscriber unit located in the first spot beam the steps comprising: tracking a movement of the subscriber unit relative to a satellite based coordinate system, estimating a transfer time at which to transfer the call based on the movement of the subscriber unit relative to the satellite based coordinate system, and causing the satellite to transfer the call at the estimated transfer time, wherein a boundary that is located between the first spot beam and the second spot beam is defined relative to the satellite based coordinate system, the step of estimating a transfer time comprises estimating an interval of time during which the subscriber unit will cross over the boundary, and the step of estimating an interval of time further comprises repeatedly adjusting the interval of time until the interval of time conforms to a desired level of accuracy.

5. A method as defined in claim 4 wherein the interval of time ends at an endpoint and wherein the step of adjusting the interval comprises: calculating a position of the subscriber unit at the endpoint of the interval; comparing the position of the subscriber unit at the endpoint to the boundary to determine whether the subscriber unit has crossed over the boundary during the interval; substituting a new value for the endpoint of the interval if the subscriber unit has not passed over the boundary during the interval; and narrowing the interval if the subscriber unit has passed over the boundary during the interval; repeating the steps of calculating, comparing, substituting and narrowing until the interval of time conforms to a desired level of accuracy.

6. A method as defined in claim 5 wherein the boundary is located equidistant from a center of the first spot beam and from a center of the second spot beam, and wherein the step of comparing comprises comparing the distance between the position of the subscriber unit at the endpoint and the center of the first spot beam to the distance between the position of the subscriber unit at the endpoint and the center of the second spot beam.

7. A method for determining when to transfer a call associated with a first satellite to a second satellite in a satellite communication system wherein the call is being transmitted from the first satellite to a subscriber unit located in a first footprint, the first footprint being generated by the first satellite, the method comprising of the steps of: a) estimating a time at which to transfer the call; b) determining the position of the subscriber unit relative to a satellite based coordinate system at the estimated time; c) calculating an angle of elevation using the position of the subscriber unit and a position of the satellite; d) comparing the angle of elevation to a threshold angle to determine whether the subscriber unit is moving out of the first footprint into a second footprint, the second footprint being generated by the second satellite; e) substituting a new time for the estimated time, if the angle of elevation is not less than the threshold angle and thereafter repeating steps b-d; f) causing the first satellite to transfer the call to the second satellite when the angle of elevation is less than the threshold angle.

8. A method as defined in claim 7 wherein the step of estimating the time at which to transfer the call and the step of determining the position of the subscriber unit at the estimated time comprise using a velocity of the satellite and an orbital path of the satellite.